1. Field of the Invention
The present invention relates to a process for coating a photoresist composition onto a substrate. In particular, the present invention relates to a process of applying atomized droplets of photoresist composition, preferably by means of an ultrasonic atomizer nozzle, onto a substrate (e.g. silicon wafer) and then forming a film of uniform thickness of photoresist solids by spinning the coated substrate.
2. Description of the Prior Art
Photoresist compositions have been applied to substrates by a variety of conventional techniques such as dip coating, roller coating, pressurized spray coating and spin coating. When the substrate is a wafer for making semiconductor devices, the present preferred method is spin coating.
By this prior art method, the photoresist composition is dispensed on the wafer while it is stopped (creating a puddle) or while it is spinning slowly. After the dispensing step, the wafer is spun at a higher rate and the resist spreads over the wafer surface due to the centrifugal force created by the spinning motion. Most of the dispensed resist is spun off of the edge of the wafer and recovered in a cup below the wafer. A film of uniform thickness of the photoresist solids (i.e. the photoactive polymer material minus most of the solvent in the original photoresist composition remains on the surface of the wafer. Then the wafer is removed from the spinning chuck for further processing.
The parameters of conventional spin coating of wafers are discussed in detail by K. Skidmore in "Applying Photoresist for Optimal Coatings" in the Feb. 1988 issue of Semiconductor International at pages 56-62. This article is incorporated herein by reference in its entirety.
One major problem of spin coating photoresist compositions onto substrates is that most of the dispensed photoresist material does not remain on the wafer. This results in either the waste of this excess photoresist material or in requiring reformulation and recycle of that material. Either alternative drastically raises the overall cost of applying the photoresist.
Separately, the technique of applying atomized droplets of material by means of an ultrasonic spray nozzle onto a substrate has been known.
Ultrasonic spray nozzles have been manufactured by Sono-Tek Corporation of Poughkeepsie, New York. Their ultrasonic nozzles have been described in U.S. Pat. Nos. 4,153,201; 4,301,968; 4,337,896; 4,352,459; 4,541,564; 4,642,581; and 4,723,708; all which are assigned to Sono-Tek and are incorporated herein by reference in their entireties.
Such ultrasonic spray nozzles have been used to apply coatings of many materials onto substrates. For example, such nozzles have been employed to apply a gelatine layer over spin-coated photoresist layer to stabilize the photoresist layer against distortion or degradation by heat generated during subsequent etching, ion implantation processes and the like. See U.S. Pat. No. 4,806,455, which issued to Nancy C. Labianca on Feb. 21, 1989 and is assigned to MacDermid Inc.
However, prior attempts to employ ultrasonic spray nozzles alone for applying photoresist coatings have been unsatisfactory due to the inability to produce a film of uniform thickness.
Accordingly, a need exists in the photoresist application art to improve upon current spin-coating appication techniques to reduce the present high photoresist dispensed volume per wafer. It is believed that the present invention which uses a combination of atomized spraying with spin-coating has the potential for reducing the amount of dispensed photoresist volumes by at least five--to ten--fold yet achieving suitable photoresist solid films of uniform thickness without a waste of most of the dispensed photoresist now commonly experienced by spin coating processes.